1. Field of the Invention
The present invention relates generally to nullification of interference signals, and more particularly to nullification of interference signals in a receiver system utilizing post-correlation temporal nulling.
2. Description of the Prior Art
A number of different techniques are used to decrease the effects of interference signals, particularly jamming signals, in receivers. Interference signals may be due to jamming signals, reflection of the transmitted signal or interference from other signals. Jamming signals are often narrowband such as continuous wave (CW-sinusoidal), modulated or narrowband noise like in nature and are designed to disturb the operation of the receiver. Countering the effects of jamming signals in global positioning system (GPS) receivers, which require the reception of numerous signals to derive a location from satellites at random locations, is particularly difficult.
Various techniques have been used to decrease the interference level in such receivers. One such technique for minimizing the effects of interference signals, particularly caused by jamming schemes, is adaptive antenna spatial nulling. Adaptive spatial nulling rejects interference signals by reducing the directional sensitivity of the receiver antenna in the direction of the interfering signal. Another well known adaptive nulling technique utilizes an adaptive transversal filter in the form of a tapped delay line for "pre-correlation" temporal nulling for canceling narrowband jammers.
Use of the above described temporal nulling technique has various drawbacks, particularly when used with a GPS receiver, such as Assignee's Anti-Jam GPS Receiver (AGR), which utilizes a "post-correlation" nulling approach for implementing adaptive spatial interference cancellation. In particular, to use the above described technique with the AGR receiver, a transversal filter must be positioned in front of each antenna processing channel. For example, for a receiver adapted to receive signals from a five-element antenna array, five independent transversal filters would have to be utilized, one filter being placed after each antenna element. The increased cost and space requirements associated with such an implementation renders the above approach impractical in most cases.
"Post-correlation" temporal nulling has not been successfully implemented in the past due to the fact that after pseudo-noise (PN) code spreading in a PN receiver, a narrowband jammer, e.g., continuous wave (CW) interference, is "spread out" such that its waveform is transformed into a Gaussian noise waveform in the receiver baseband.
What is needed therefore is a post-correlation temporal nulling system for use with the AGR receiver which is practical, economical and space efficient.